PSI - Issue 48

Oleh Yasniy et al. / Procedia Structural Integrity 48 (2023) 149–154 Yasniy et al/ Structural Integrity Procedia 00 (2023) 000–000

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Earlier, under the supervision of Prof. P.V. Yasnii, a study of the stress-strain diagram of AMg6 alloy under quasi-static tension under mild loading conditions was conducted, which revealed changes in the microstructure of the material and proposed models that establish a relationship between them. Similar studies were conducted on the creep and dynamic creep processes of AMg6 alloy by Yasnii et al. (2004, 2004).

р

 p

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IІ

ІV

р ст

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Fig. 2. Creep curve of AMg6 alloy: І – strengthening area, ІІ – steady creep area, ІІІ – jump-like creep area, ІV – failure area.

Analysis of microstructure shows that after creep tests, the AMg6 alloy contains fractured and still continuous dispersoids. Their distribution, depending on the shape factor, is described by a dynamic tensile histogram. Therefore, for modeling purposes, we assume that the process of jump creep, similar to jump tensile deformation, is caused by the cracking of dispersoids in the material volume. Based on this assumption, a methodology for predicting the initiation of jump creep depending on the fraction of fractured inclusions was proposed by Fedak (2003). The average values of the relative prediction error for the parameters of initiation of instantaneous creep growth, namely, creep strain р ст , time t ст , and the value of the incremental strain of the jump  р, respectively, are 21.4%, 29.7%, and 45.3%. According to the proposed model, the data on the strain and time of jump initiation during creep are satisfactorily consistent. A larger data scatter is observed in the prediction of jump-like strain increases. This can be explained by homogenization processes in the material volume during creep. The redistribution of the stress-strain state in the volumes of heterogeneous material structure leads to a less predictable value of local strengthening of material and, accordingly, a less predictable value of strain increment. 3. Experimental Approach As a development of these studies, the ANSYS software package was used to predict jump creep. Applying this software, the groups of finite element models were developed to determine the main regularities of influence of structural heterogeneity parameters of the modeled medium on the stress-strain state by Yasniy et al. (2010). The creep and dynamic creep deformation of AMg6 alloy were calculated by FEM. The simulation was performed in a plane-deformed state on the computational model. During the calculations, the options for creep and fracture of the structural components of the model were activated when they reached critical stresses. A plane 82 element was used to construct the finite element mesh. It was found that creep under static and cyclic loading is accompanied by the destruction of inclusions. Under dynamic creep, the high-frequency component of the load causes higher stresses on the inclusions compared to creep, which increases the number of destroyed inclusions. The fracture of inclusions causes a redistribution of stresses and strains in the model and intensifies creep under cyclic loading compared to static loading. The accumulated strain under dynamic creep is greater than under static creep at the same level of maximum load. This can be explained by the intensive redistribution of stress and strain fields under cyclic loading and the corresponding increase in the maximum stress on the inclusion and in the matrix material. Due to the increase in local stresses in the computational model under dynamic creep conditions, the limit state of the inclusions and the matrix is reached faster. The damageability of the material increases in general, and, accordingly, the total creep strain increases significantly compared to creep. The minimum creep stress at which the fracture of inclusions of the model material begins is *  =229 MPa, according to experimental data. It should be noted that the